JP6248748B2 - Roughing end mill - Google Patents

Description

  The present invention relates to a luffing end mill in which a wavy outer peripheral blade that is uneven in the radial direction of the end mill body as viewed from the end mill rotation direction is formed on the outer periphery of the end portion of the end mill body that is rotated about an axis.

  As such a luffing end mill, for example, in Patent Document 1, a plurality of blade grooves having a twist in the rotation direction are provided at one end of the shank, and a rotation locus centering on the rotation axis is a convex arc between the plurality of blade grooves. And a wavy roughing blade having a concave arc portion. In such a roughing end mill, the phase of the waveform of each of the plurality of wavy roughing blades adjacent to each other in the circumferential direction of the end mill body is shifted, and cutting is performed exclusively by roughing the portion where the roughing blade is convex. The cutting resistance can be reduced by dividing and generating chips.

  In Patent Document 2, such a luffing end mill is cut using a grindstone having a grinding surface having a cross-sectional shape corresponding to the waveform formed by the cutting blade at intervals based on the pitch of the waveform. A manufacturing method of a luffing end mill that is manufactured by grinding the cutting blade while moving along the blade is described. In a roughing end mill having a wavy shape in which the outer peripheral edge is uneven in the radial direction of the end mill main body, such a grindstone grinds the outer peripheral flank face connected to the rear side in the end mill rotation direction of the outer peripheral edge into a corrugated cross section.

JP 2008-137121 A Japanese Patent No. 4211655

  By the way, the outer peripheral flank surface of such an end mill is usually directed toward the inner peripheral side of the end mill main body as it goes from the outer peripheral blade to the rear side in the end mill rotation direction, that is, in the cross section perpendicular to the axis of the end mill main body. The clearance angle is generally given so as to gradually move away from the circle formed around the axis, and this also applies to the roughing end mill. For this reason, in the luffing end mill, the outer peripheral flank is formed by backing the grindstone as described above from the outer peripheral blade toward the inner peripheral side of the end mill main body toward the rear side in the end mill rotation direction.

  On the other hand, even with such a roughing end mill, if heavy cutting with a large cutting depth and feed amount is performed, the resistance acting on the end mill main body may increase and chatter vibration may occur. In order to prevent such chatter vibration, a margin part with a clearance angle of 0 ° is formed on the outer peripheral blade side of the outer peripheral flank surface, or the clearance angle of the outer peripheral flank surface on the outer peripheral blade side is set to the rear side in the end mill rotation direction. It is conceivable that the marginal portion or the outer peripheral flank face on the outer peripheral blade side is pressed against the processing surface of the work material to provide a burnishing effect and improve the rigidity of the outer peripheral blade.

  However, in order to make the clearance angle different between the outer peripheral flank side of the outer peripheral flank and the rear side in the end mill rotation direction as described above, in the manufacturing method using the grindstone described in Patent Document 2, in particular, Grinding must be performed with different angles of inclination with respect to the outer peripheral edge, and in the roughing end mill manufactured in this way, the shape and dimensions of the waveform formed by the outer peripheral edge when viewed from the end mill rotation direction are not uniform. There is a risk that. And when the shape and dimensions of the corrugation formed by the outer peripheral blade are non-uniform in this way, a portion that is largely cut into the work material is generated even in the portion where the outer peripheral blade is convex. Defects and abnormal wear will occur at various locations.

  The present invention has been made under such a background. As described above, chatter vibration is generated even in heavy cutting while suppressing the non-uniform shape and size of the waveform formed by the outer peripheral blade. An object of the present invention is to provide a luffing end mill capable of preventing the above.

To solve the above problems, in order to achieve the above object, the present invention is the first, the distal end outer periphery of the end mill body which is rotated around an axis extending in the rear end side from the tip of the end mill body An outer peripheral blade is formed, and the outer peripheral blade is formed in a wavy shape that is uneven in the radial direction of the end mill body as viewed from the end mill rotation direction, and at least the outer peripheral blade is convex on the outer peripheral side of the end mill body. the end mill rotating direction rear side of the peripheral relief surface connecting to the protruding portion made, the outer peripheral cutting edge at the convex portion toward the the end mill rotating direction rear side have unit Kireagari approach the circle formed around the axis is formed The cut-out portion is formed on the rear side in the end mill rotation direction of the outer peripheral flank surface connected to some of the convex portions of the one outer peripheral blade. , The end mill rotating direction rear side of the peripheral relief surface connecting to the rest of the protrusion, characterized in that not the Kireagari portion is formed.
Further, according to the second aspect of the present invention, an outer peripheral blade extending from the front end of the end mill main body to the rear end is formed on the outer periphery of the front end of the end mill main body that is rotated around the axis. The end mill body is formed in a wavy shape that is uneven in the radial direction of the end mill body, and at least the outer peripheral flank of the outer peripheral blade that is convex to the outer peripheral side of the end mill body on the rear side in the end mill rotation direction. Is formed with a rounded-up portion that approaches the circle formed by the outer peripheral blade around the axis in the convex portion as it goes rearward in the end mill rotation direction, and a plurality of outer peripheral blades are formed in the end mill body. Of these, the above-mentioned cut-out portion is formed on the rear side in the end mill rotation direction of the outer peripheral flank surface connected to at least a portion of the convex portions of the outer peripheral blades. Together we are, in the end mill rotating direction rear side of the peripheral relief surface connecting to the convex portion of the remaining peripheral cutting edge, characterized in that not the Kireagari portion is formed.

  Therefore, in the roughing end mill configured in this way, a rounded-up portion is formed on the rear side in the end mill rotation direction of the outer peripheral flank that is continuous with the convex portion that is convex on the outer peripheral side of the end mill body among the outer peripheral blades, Since this rounded-up portion is formed so that the outer peripheral blade approaches the circle formed around the axis in the convex portion as it goes rearward in the end mill rotation direction, this rounded-up portion is cut into the work material during heavy cutting. It is possible to obtain a burnishing effect by pressing against the processed surface, and to secure the thickness of the end mill body on the rear side in the end mill rotation direction of the outer peripheral blade, thereby improving the rigidity. That is, by this cut-up portion, it is possible to obtain the same effect as the case where a margin portion and a portion with a small clearance angle are formed on the outer peripheral flank surface, so that occurrence of chatter vibration can be suppressed even in heavy cutting, Smooth and highly accurate processing can be performed.

  In order to form such a cut-off portion, for example, when grinding the outer peripheral flank while backing with the above-described grindstone, the backing is stopped just before the grindstone comes off from the outer peripheral flank, and the grindstone is left as it is. Can be separated from the outer flank. In this case, the circumferential shape of the grindstone is transferred to a portion of the outer peripheral flank on the rear side in the end mill rotation direction, and the outer peripheral blade approaches the circle formed around the axis as the outer peripheral flank faces toward the rear in the end mill rotation direction. Will be cut up like this.

Here, when such a cut-up portion is formed on the rear side in the end mill rotation direction of the outer peripheral flank face connected to all the convex portions of the outer peripheral blade , for example, the cut-off formed by the method described above It is difficult to accurately control the size of the portion in the axial direction and the amount of separation from the circle formed by the outer peripheral blade around the above-mentioned axis, thus forming a rounded portion on the outer peripheral flank of all the convex portions. Sometimes, if the size of these rounded-up parts is too small or the distance is too large, chatter vibration may not be sufficiently prevented, while the rounded-up parts are too large or If it also protrudes to the outer peripheral side, the cut-up portion is excessively pressed against the processing surface during heavy cutting, which may increase excessive cutting resistance.

Therefore, in the present invention, firstly , the above-mentioned raised portion is formed on the rear side in the end mill rotation direction of the outer peripheral flank surface connected to a part of the convex portions of one outer peripheral blade, and the remaining convex portions. By controlling the number of cut-off portions so as not to be formed on the rear side in the end mill rotation direction of the outer peripheral flank face continuous with the portion, generation of chatter vibration and an increase in cutting resistance are prevented .

In the present invention, the second, if it is formed the outer peripheral edge of the plural rows in the end mill body, the end mill rotating direction behind the peripheral relief surface connecting to at least a portion of the convex portion of the peripheral cutting edge of these portions The number of the cut-off portions is also controlled by forming the above-mentioned cut-out portions on the side and not forming the cut-out portions on the rear side in the end mill rotation direction of the outer peripheral flank surface connected to the convex portions of the remaining outer peripheral blades. I am doing so.

  As described above, according to the present invention, chatter vibrations are generated even in heavy cutting while suppressing the occurrence of defects and abnormal wear due to the non-uniform shape and size of the waveform formed by the outer peripheral blade. This makes it possible to perform smooth and highly accurate cutting.

It is a perspective view which shows the reference example in describing embodiment of this invention. It is the front view which looked at the reference example shown in FIG. 1 from the axial direction front end side. It is a side view of the reference example shown in FIG. 1A is a view of the outer peripheral blade of the reference example as viewed from the end mill rotation direction along a plane perpendicular to the axis of the end mill body, and FIG. 1B is an end mill body along the plane perpendicular to the axis of the end mill body. It is the figure seen from the outer peripheral side. It is ZZ expanded sectional view in Drawing 4 (a) and (b). It is the side view seen from the end mill rotation direction back side which shows the positional relationship of the outer periphery blade and grindstone at the time of manufacturing the end mill of the reference example shown in FIG. It is sectional drawing orthogonal to the axis line of the end mill main body which shows the positional relationship of the outer periphery blade and grindstone at the time of manufacturing the end mill of the reference example shown in FIG. 1A is a view of an outer peripheral blade according to an embodiment of the present invention when viewed from the end mill rotation direction along a plane perpendicular to the axis of the end mill body, and FIG. 2B is a view of the end mill body along the plane perpendicular to the axis of the end mill body. It is the figure seen from the outer peripheral side.

1 to 5 show reference examples for explaining the embodiment of the present invention. In this reference example , the end mill body 1 is formed in a substantially cylindrical shape with a center line about the axis O by a hard material such as cemented carbide, and the rear end portion (the upper right portion in FIG. 1 and the upper portion in FIG. 3) is The shank portion 2 remains in a cylindrical shape, and the tip portion (lower left portion in FIG. 1, lower portion in FIG. 3) is a cutting blade portion 3. In such a luffing end mill, the shank portion 2 is gripped by the main spindle of the machine tool and rotated in the end mill rotation direction T around the axis O, and is usually fed in a direction perpendicular to the axis O to be cut into the work material. Process it.

A chip discharge groove 4 extending from the front end of the end mill body 1 toward the rear end side is formed on the outer periphery of the cutting edge portion 3, and the outer peripheral side edge of the wall surface of the chip discharge groove 4 facing the end mill rotation direction T. The part is formed with an outer peripheral blade 6 having the wall surface as a rake face and the outer peripheral surface of the cutting edge part 3 intersecting the rake face as an outer peripheral flank face 5. In this reference example , four chip discharge grooves 4 are formed in the cutting blade portion 3 at intervals in the circumferential direction. Accordingly, the outer peripheral blade 6 is also formed at four intervals in the circumferential direction.

As shown in FIG. 3, a gash 7 having a substantially V-shaped groove shape is formed at the tip of each chip discharge groove 4, and the tips of the wall surfaces of these gashes 7 facing the end mill rotation direction T are formed. At the edge, a bottom blade 8 having a rake face as the wall surface is formed so as to extend from the tip of each outer peripheral blade 6 to the inner peripheral side. The roughing end mill of the present reference example is a radius type roughing end mill in which the bottom blade 8 intersects the outer peripheral blade 6 via a convex curved corner blade such as an arc.

  Here, as shown in FIG. 2, the bottom blade 8 extends from the tip of the outer peripheral blade 6 to a long bottom blade (bottom blade extending vertically in FIG. 2) 8 extending in the vicinity of the axis O and to a position away from the axis O. Short bottom blades (bottom blades extending left and right in FIG. 2) 8 are alternately formed in the circumferential direction. The distance from the long bottom blade 8 to the short bottom blade 8 adjacent in the end mill rotation direction T is larger than the distance from the short bottom blade 8 to the long bottom blade 8 adjacent in the end mill rotation direction T. .

Each chip discharge groove 4 is formed to be twisted to the rear side in the end mill rotation direction T around the axis O as it goes from the front end to the rear end side of the end mill main body 1. 1 is formed in a spiral shape that twists toward the rear side in the end mill rotation direction T around the axis O as it goes from the front end to the rear end side. In this reference example , the twist angles of the spirals formed by the four outer peripheral blades 6 are formed to be equal to each other. Therefore, the circumferential interval between the four outer peripheral blades 6 is the outer peripheral blade that is continuous with the long bottom blade 8. The distance from 6 to the outer peripheral blade 6 adjacent to the end mill rotation direction T is larger than the distance from the outer peripheral blade 6 connected to the short bottom blade 8 to the outer peripheral blade 6 adjacent to the end mill rotation direction T.

Further, the outer peripheral flank 5 of these outer peripheral blades 6 is formed so as to be uneven in the radial direction (inner and outer periphery of the end mill body 1) from the front end to the rear end side of the end mill main body 1. As shown in FIG. 4A, the outer peripheral blade 6 is also formed in a wave shape that is uneven in the radial direction with respect to the axis O of the end mill body 1 as seen from the end mill rotation direction T. In this reference example , the waveform formed by the outer peripheral edge 6 has a fixed period (a convex arc that is convex toward the outer peripheral side of the end mill body 1 and a concave arc that is concave toward the inner peripheral side when viewed from the end mill rotation direction T. The pitch is formed in a continuous shape.

Further, the shape and size of the waveform formed by each outer peripheral blade 6, that is, the period and the amplitude are equal to each other between the four outer peripheral blades 6, but the phase of this waveform is the peripheral blade 6 adjacent in the circumferential direction. Are shifted in the direction of the axis O. In the present reference example , the phase is sequentially shifted to the front end side in the axis O direction by 1/4 of one cycle of the waveform between one outer peripheral blade 6 and the outer peripheral blade 6 adjacent to the rear side in the end mill rotation direction T. Yes.

The outer peripheral flank 5 has at least a portion of the outer peripheral blade 6 connected to the convex portion 6a protruding toward the outer peripheral side of the end mill main body 1 on the rear side in the end mill rotational direction T and on the rear side in the end mill rotational direction T. As it goes, a rounded-up portion 5a that approaches a circle C formed by the outer peripheral blade 6 around the axis O is formed in the convex portion 6a. Here, in the present reference example , as shown in FIG. 4B, a cut-out portion 5 a is formed on the rear side in the end mill rotation direction T of the outer peripheral flank 5 connected to all the convex portions 6 a of the outer peripheral blade 6.

That is, the peripheral flank surface 5 in a cross section perpendicular to the axis O, as it goes from the peripheral cutting edge 6 on the rear side of the end mill rotating direction T, while drawing a convex curve which is convex to the outer peripheral side of the reference example in the end mill body 1 mill After going to the inner peripheral side of the main body 1, the cut-out portion 5a shown by hatching in FIG. 4B is formed so as to cut to the outer peripheral side while forming a concave curve in contact with the convex curve. An outer peripheral blade 6 adjacent to the rear side of the end mill rotation direction T of the outer peripheral blade 6 connected to the outer peripheral flank 5 is formed further on the rear side in the end mill rotation direction T of the cut-out portion 5a of the outer peripheral flank 5. Wall surfaces facing the end mill rotation direction T rear side of the chip discharge groove 4 are continuous so as to intersect an obtuse angle.

  Such a cut-off portion 5a is formed by grinding the outer peripheral flank 5 with a disc-shaped grindstone G having an outer peripheral surface having a cross-sectional shape obtained by inverting the irregularities formed by the outer peripheral blade 6 as shown in FIG. 7, as shown by the arrow B in FIG. 7, as shown by the arrow B in the outer peripheral flank 5, the grindstone G moves toward the rear side in the end mill rotation direction T on the outer peripheral blade 6 side. To the inner peripheral side of the end mill body 1 so as to make a backing, and when the grindstone G reaches the rear side in the end mill rotation direction T of the outer peripheral flank 5, the movement is stopped and the grindstone G is separated from the outer peripheral flank 5 as it is. You can do it. By transferring the circumference formed by the outer peripheral surface of the grindstone G, the cross section of the cut-out portion 5a is formed in the concave curve as described above.

  In the luffing end mill configured as described above, the rounded portion 5a is formed on the rear side in the end mill rotation direction T of the outer peripheral flank 5 and the outer peripheral flank 5 is formed in a circle C around the axis O. Therefore, it is possible to obtain a burnishing effect by rubbing the cut-up portion 5a against the work surface of the work material formed by the outer peripheral blade 6 particularly in heavy cutting with a large feed and cutting. The end mill body 1 can be stably supported during the cutting process.

  Further, by forming the cut-off portion 5a in this way, it is possible to secure a large thickness of the end mill body 1 on the rear side in the end mill rotation direction T of the outer peripheral blade 6 and to improve the rigidity of the outer peripheral blade 6. Thus, chatter vibrations can be prevented from occurring in the end mill body 1. Therefore, according to the luffing end mill having the above-described configuration, it is possible to stably and smoothly perform high-precision cutting even with the above-described heavy cutting.

  On the other hand, the burnishing effect and high rigidity can be obtained by the cut-out portion 5a on the rear side in the end mill rotation direction T of the outer peripheral flank 5 away from the outer peripheral blade 6 in this way. It is not necessary to form a margin part or a part with a small clearance angle in the part connected to. Therefore, it is not necessary to make the inclination angle of the grindstone G different in order to form such a portion, and the outer peripheral blade 6 has a uniform shape and size of the waveform when viewed from the end mill rotation direction T, and is missing. And abnormal wear can be prevented.

Here, the distance from the circle C of the rounded portion 5a that is close to the circle C formed by the outer peripheral blade 6 around the axis O in this way (in this reference example , the rounded portion 5a in the cross section perpendicular to the axis O). And the shortest distance from the intersection of the chip discharge groove 4 adjacent to the rear side in the end mill rotation direction T of the cut-up portion 5a to the wall surface facing the rear side in the end mill rotation direction T to the circle C. Hereinafter, referred to as a drop amount. D may be 0 if the cut-out portion 5a does not protrude to the outer peripheral side from the circle C, that is, the outermost periphery of the cut-out portion 5a is located on the circle C as shown in FIG. Also good. On the other hand, if the drop amount D is too large, the effects as described above may not be obtained. Therefore, the drop amount D is 0.03 times the diameter of the circle C (the outer diameter of the outer peripheral blade 6). The following is desirable.

By the way, in this reference example, such a cut-up portion 5a is formed on the rear side in the end mill rotation direction T of the outer peripheral flank 5 that is continuous with all the convex and concave portions 6a formed by the outer peripheral blade 6. For this reason, while improving the rigidity with respect to the convex part 6a of all the outer periphery blades 6, while the convex part 6a is cut in the work material, it always supports the end mill main body 1 stably. It becomes possible.

  However, as described above, the cut-up portion 5a is in sliding contact with the work surface of the work material and exhibits a burnishing effect, and thus the cut-up portion 5a is continuous with the convex portions 6a of all the outer peripheral blades 6. If it is formed on the outer peripheral flank 5, the cutting resistance may be excessively increased due to the effect of cutting off the chips by the wavy outer peripheral blade 6. In such a case, the burnishing effect may be weakened by increasing the drop amount D or reducing the area of the cut-off portion 5a, but it is difficult to accurately control these.

Therefore, in such a case, instead of controlling the drop amount D and the area of the cut-out portion 5a, the cut-out portion 5a is arranged on one outer periphery as in the embodiment of the present invention shown in FIG. Of the convex portions 6a of the blade 6, the outer peripheral flank 5 connected to a part of the convex portions 6a is formed on the rear side in the end mill rotation direction T, and the outer peripheral flank 5 connected to the remaining convex portions 6a on the end mill rotational direction T rear side. The number of cut-out portions 5a may be controlled without forming them. In this embodiment , the same reference numerals are assigned to portions common to the reference example, and the description thereof is omitted.

Here, in this embodiment , the cut-out portion 5a is formed only on the rear side in the end mill rotation direction T of the outer peripheral flank 5 connected to every other convex portion 6a along one outer peripheral blade 6. Also, the cut-out portion 5a is not formed on the rear side in the end mill rotation direction T of the outer peripheral flank 5 connected to every other remaining convex portion 6a between these convex portions 6a, similarly to the outer peripheral blade 6 side. It forms so that it may go to the inner peripheral side of the end mill main body 1 as it goes to the end mill rotation direction T back side. The other peripheral blades 6 have the same configuration.

Therefore, according to the luffing end mill of such an embodiment , the burnishing effect does not occur in the outer peripheral flank 5 where the cut-out portion 5a is not formed, and thus the increase in cutting resistance of the end mill body 1 as a whole can be suppressed. it can. Further, since the cut-out portions 5a are arranged at equal intervals on the outer peripheral flank 5 of one outer peripheral blade 6, the strength of the burnishing effect does not partially occur and the cutting resistance does not increase or decrease irregularly. However, the cut-out portions 5a may be formed at unequal intervals, or different intervals or different numbers of the cut-out portions 5a may be formed between the outer peripheral blades 6.

  Further, instead of forming the cut-out portion 5a only on the outer peripheral flank 5 connected to a part of the convex portions 6a of one outer peripheral blade 6, a plurality of outer peripheral blades 6 are formed on the end mill body 1. In this case, a cut-out portion 5a is formed on the rear side in the end mill rotation direction T of the outer peripheral flank 5 connected to at least a part of the convex portions 6a of some of the outer peripheral blades 6 and the remaining outer peripheral blades 6 The burnishing effect is adjusted by controlling the number of the raised portions 5a so as not to form the raised portions 5a on the rear side in the end mill rotation direction T of the outer peripheral flank 5 connected to the convex portion 6a. Also good.

For example, when even-numbered outer peripheral blades 6 are formed as in the above-mentioned reference example , outer peripheral blades 6 are alternately formed in the circumferential direction with outer peripheral blades 6 formed on the outer peripheral flank 5 and outer periphery not formed. A blade 6 may be provided, and in addition to this, an outer peripheral blade 6 having a rounded portion 5a formed on the outer peripheral flank 5 and an outer peripheral blade 6 not formed are provided for every predetermined number of strips. Further, these outer peripheral blades 6 may be arranged irregularly. The outer peripheral edge 6 Kireagari portion 5a is formed, which may be the Kireagari portion 5a in the end mill rotating direction T rear side of all the convex portions 6a as Reference Example is formed, the above-described embodiment Thus, the cut-out portion 5a may be formed only on the outer peripheral flank 5 that continues to some of the convex portions 6a.

DESCRIPTION OF SYMBOLS 1 End mill main body 2 Shank part 3 Cutting blade part 4 Chip discharge groove 5 Outer peripheral flank 5a Round-up part 6 Outer peripheral blade 6a Convex part of outer peripheral blade 6 7 Gash 8 Bottom blade O End mill main body 1 axis T End mill rotation direction C Outer peripheral blade Circle D formed by the convex portion 6a of the axis 6 around the axis O (amount of separation from the circle C of the cut-out portion 5a in the cross section orthogonal to the axis O)

Claims (2)

An outer peripheral blade extending from the front end of the end mill main body to the rear end is formed on the outer periphery of the end mill main body rotated around the axis, and the outer peripheral blade extends in the radial direction of the end mill main body as viewed from the end mill rotation direction. It is formed in a corrugated wave shape, and at least the outer peripheral flank of the outer peripheral blade connected to the convex portion projecting to the outer peripheral side of the end mill body is directed to the rear side in the end mill rotational direction on the rear side in the end mill rotational direction. In accordance with the convex portion, a rounded-up portion is formed that approaches the circle formed by the outer peripheral blade around the axis ,
The cut-up portion is formed on the rear side in the end mill rotation direction of the outer peripheral flank surface connected to some of the convex portions of the one outer peripheral blade, and the outer peripheral flank surface connected to the remaining convex portions. A roughing end mill , wherein the above-mentioned cut-out portion is not formed on the rear side in the end mill rotation direction . An outer peripheral blade extending from the front end of the end mill main body to the rear end is formed on the outer periphery of the end mill main body rotated around the axis, and the outer peripheral blade extends in the radial direction of the end mill main body as viewed from the end mill rotation direction. It is formed in a corrugated wave shape, and at least the outer peripheral flank of the outer peripheral blade connected to the convex portion projecting to the outer peripheral side of the end mill body is directed to the rear side in the end mill rotational direction on the rear side in the end mill rotational direction. In accordance with the convex portion, a rounded-up portion is formed that approaches the circle formed by the outer peripheral blade around the axis,
A plurality of the outer peripheral blades are formed on the end mill body, and among these, the cut-out portion is formed on the rear side in the end mill rotation direction of the outer peripheral flank surface connected to at least some of the convex portions of the outer peripheral blades. The roughing end mill is characterized in that the rounded end mill is not formed on the rear side in the end mill rotation direction of the outer peripheral flank face connected to the convex portion of the remaining outer peripheral blade .

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